a) Field of the Invention
The present invention relates to a microwave-millimeter wave circuit device and a method for manufacturing the same, and more particularly to a microwave-millimeter wave circuit device having a coplanar wiring and a method for manufacturing the circuit device.
b) Description of the Related Art
A monolithic microwave-millimeter wave integrated circuit device (MMIC) is known as a semiconductor IC device used in a high frequency band associated with microwaves, millimeter waves, etc. The use of a high frequency is required for transmission, reception and the like. In signal processing, however, operations at such a high frequency are not required. In view of this, the method of forming circuits which operate at a particularly high frequency on IC chips and bonding the IC chips onto a circuit board is adopted. In most cases, an MMIC is formed as such an IC chip.
The mobility of charge carriers in a semiconductor is restricted by the material of the semiconductor. Generally speaking, the mobility of carriers in a compound semiconductor like GaAs and InP is higher than that of carriers in Si. The compound semiconductor, therefore, is more suitable than Si in forming a high frequency IC.
A semiconductor element whose parasitic capacitance is small is preferred as one which operates in a high frequency band. In the case of the compound semiconductor, since a semi-insulating region can be formed therein by Cr doping, oxygen ion implantation, etc., a semiconductor element employing dielectric isolation, not pn isolation, can be provided. By adopting the chemical compound and conducting the dielectric isolation, a semiconductor device whose attendant capacitance is considerably small can be realized.
A 77 GHz radar, for example, is available as a millimeter wave circuit device. When forming the entirety of the radar on the compound semiconductor, a compound semiconductor substrate having a large area is required, because its transmission/reception antenna occupies a large area. This results in the cost of the entire device being extremely high. In order to reduce the manufacturing cost, it is preferable to form an antenna on a dielectric substrate made of an inexpensive material and to form only a circuit section, which needs to operate at a high frequency, as an MMIC made of the compound semiconductor, and to bond the MMIC onto the substrate on which the antenna has been formed.
In general, the compound semiconductor substrate has a dielectric constant of 10 or greater. Signals, transmitted through a wiring formed on the substrate with such a high dielectric constant, have a short wavelength. In the case of a frequency of 77 GHz, signals transmitted through coplanar lines, for example, have a wavelength of approximately 1.6 mm.
The geometric dimensions of a semiconductor active element formed on the compound semiconductor substrate tend to decrease as its performance is improved. A minor variation in the shape of the semiconductor element results in a considerable variation in the input/output impedance of the semiconductor active element.
When the input/output impedance of the semiconductor active element as formed differs from the design value, the dimensions of a peripheral circuit element, in particular, a matching circuit, need to be changed. Any design change will be possible if a semiconductor IC device is remanufactured from the beginning. However, the semiconductor substrate on which the semiconductor element, etc. have been formed has to be abandoned. This makes the manufacturing cost high.
In the case of a coplanar wiring, a ground conductor and a signal wiring are arranged in the same plane. The signal wiring separates the ground conductor into parts. Under this condition, it is difficult to keep the potential of the ground conductor uniform. The potential of the ground conductor is unstable especially when the ground conductor has such a width and a length that the conductor itself has impedance.